The present invention relates to a heater for heating an object to be heated such as gas and liquid, a glow plug for diesel engine and a water heater for heating water.
As a heater for use in a diesel engine there is a glow plug. This glow plug comprises a cylindrical metal shell, an metal sheath forming a heat-generating portion, a heating coil as a heating element provided in the metal sheath, a center pole extending from the opening of the metal sheath to the interior of the metal sheath and connected to the heating coil directly or indirectly with a control coil, and an insulating material packed in the interior of the metal sheath. The opening of the metal sheath is closed by an elastic packing provided between the center pole and the metal sheath.
Defects of the aforementioned glow plug include expansion of the metal sheath caused by the moistening of the insulating material and oil short caused by the permeation of oil into the metal sheath.
The invention has been worked out under these circumstances. The invention provides a heater comprising a metal sheath, a heating element connected to the inner surface of the metal sheath at one end thereof, a center pole extending from the opening of the metal sheath to the interior of the metal sheath and directly or indirectly connected to the heating element and an insulating material packed in the interior of the metal sheath, the opening of the metal sheath being closed by sealing the gap between the metal sheath and the center pole with an elastic packing, wherein the axial length of the elastic packing between the metal sheath and the center pole is predetermined to be not smaller than 2.5 mm.
By predetermining the length of the elastic packing between the metal sheath and the center pole to be not smaller than 2.5 mm, sealing properties passing a pressure cooker test (Electronic Industries Association of Japan Specification IC-121) can be obtained. Further, no oil short occurs even upon a test involving the repetition of a pattern comprising dipping of the specimen in the oil and subsequent electrical energization thereof.
Preferably, the length of the elastic packing between the metal sheath and the center pole is predetermined to be from 3 mm to 6 mm. The increase of the length of the elastic packing between the metal sheath and the center pole makes it difficult for the elastic packing to be inserted into the sealed portion. However, when the length of the elastic packing between the metal sheath and the center pole falls within the above defined range, it rarely raises a problem of deterioration of insertion properties but provides improved sealing properties giving great advantages.
The aforementioned heater preferably an engagement portion and a metal shell for interference-fitting the metal sheath at the engagement portion. The metal sheath preferably has a fixed (interfitted) portion fixed to the metal shell and a small diameter portion having a smaller diameter than the inner diameter of the engagement portion provided between the fixed portion and the opening. The axial length of the small diameter portion is preferably greater than the length of the elastic packing between the metal sheath and the center pole. In this arrangement, the elastic packing is provided at the small diameter portion. In other words, since the interior of the fixed portion in the metal sheath has no elastic packing provided therein and is filled with an insulating material harder than the elastic packing, it is further assured that the metal sheath can be interference-fitted onto the metal shell.
Further, the difference in diameter between the small diameter portion of the metal sheath and the inner diameter of the fixed portion of the metal shell is preferably from 0.02 mm to 0.5 mm. When the difference in diameter falls below 0.02 mm, it is made difficult for the small diameter portion of the metal sheath to be smoothly inserted into the engagement portion of the metal shell when pressed thereinto. On the contrary, when the difference in diameter exceeds 0.5 mm, the load at which the small diameter portion is pressed into the engagement portion is excessive, making it likely that deformation can occur. The range of difference in diameter within which the desired effect can be exerted is more preferably from 0.02. mm to 0.3 mm.
Preferably, the elastic packing doesn""t protrude from the opening of the metal sheath 3 and is received in the interior of the metal sheath 3. Further, when the elastic packing 15 protrudes from the opening of the metal sheath 3, the length of protrusion is preferably not greater than 3 mm. This is. because when the length of protrusion of the elastic packing 15 from the opening of the metal sheath 3 falls within a range of from 0 to 3 mm, the protruding elastic packing 15 can be prevented from interfering with the insertion of the metal sheath 3 into the metal shell 2.
The material of the elastic packing is preferably a silicon rubber or fluororesin because it has an excellent heat resistance as described in claim 6. The elastic packing is also preferably in the form of O-ring or ring having a rectangular section as described in claim 7. In this arrangement, the elastic packing can be easily inserted into the metal sheath. Further, the resulting uniform elastic form in the circumferential direction makes it possible to obtain high sealing properties.
Further, the invention provides a glow plug having the aforementioned heater the purpose of which is predetermined for use in diesel engine as described in claim 8. Moreover, the invention provides a water heater having the aforementioned heater the purpose of which is predetermined to heat water as described in claim 9.